problematic O2 Sensor

[astrojohn]

I've got a 2000 AWD Astro, with 315,000 miles on it. It's a very dependable vehicle, and has paid for itself many times over.
However, I recently had to replace the cat convrter, as it kept throwing a code of PO 420 (I believe).
I replaced all the O2 sensors at that time also.
I hoped never to have to deal with that code popping up again.
However, after a few days, when I first would start it up, after approx 30 seconds, the check engine light would go on, and a code of PO1035 pop up. Apparently, the heater element in the O2 sensor had or has a glitch.
This code is for the driver side, pre cat sensor. I went back to the auto parts store where I purchased the sensor, and told him of the problem. They exchanged the sensor for a new one, which was a Bosch unit.
We had figured a faulty sensor.
Well now that it's been cold out, almost every morning after starting, and running from 30 seconds, to 5 minutes, the PO 1035 code pops back up.
When I got the last sensor, the parts man, said the original type sensor was no longer available, so the one he gave me was supposed to be the replacement.
Is there a better O2 sensor that I can buy to get rid of the problem.
I was hoping that changing out the cat, (all stainless mandrel bent) and O2 sensors, would fix all my problems.
Is there something I'm missing? The wiring harness doesn't look damaged in any way, and I did use dielectric grease on the connections. HELP!!!!!!!